The present invention relates to an optical displacement sensor and an optical encoder and, more particularly, to an optical displacement sensor for detecting the displaced amount of a precision mechanism.
The general structure of an encoder, which is a typical conventional displacement sensor as a first prior art, will be described first. FIG. 28 shows the structure of a conventional laser encoder using a coherent light source and a diffraction grating scale as an example of a miniaturized low cost encoder that does not require assembly of optical parts such as a lens.
A laser encoder using a coherent light source and a diffraction grating scale is described in, for example, “Copal: Rotary encoder catalogue”. AS shown in FIG. 28, the laser encoder is constructed such that a laser beam emitted from a semiconductor laser constituting a coherent light source 1 is irradiated on a transmission type diffraction grating scale 2 to form a diffraction interference pattern 13. A specific portion of the diffraction interference pattern 13 thus formed passes through transmission slits 53 located at predetermined distances P2 so as to be detected by a photosensor 3.
FIGS. 29A to 29E show in detail the operation of the displacement sensor using the laser encoder, which is shown in FIG. 28.
First of all, the operation of the conventional displacement sensor shown in FIG. 28 will be described with reference to FIGS. 29A to 29E. The individual structural parameters shown in FIG. 29A are defined as follows:                z1: distance between the light source 1 and the surface of the scale 2 on which the diffraction grating is formed;        z2: distance between the surface of the scale 2 on which the diffraction grating is formed and the light receiving surface of the photosensor;        p1: pitch of the diffraction grating on the scale 2;        p2: pitch of a diffraction interference pattern 13 on the light receiving surface of the photosensor 3;        θx: spread angle of the light beam emitted from the light source toward the pitch of the diffraction grating on the scale 2; and        θy: spread angle of the light beam emitted from the light source in a direction perpendicular to the spread angle θx (the spread angle of the light beam represents an angle made by a pair of boundary lines 6 at which the intensity of the light beam becomes a half in a direction in which the intensity of the light beam forms a peak).        
It should be noted that the “pitch of the diffraction grating on the scale 2” represents the spatial period of a pattern formed by modulating the optical characteristics formed on the scale 2. Also, the “pitch of the diffraction interference pattern 13 on the light receiving surface of the photosensor 3” represents the spatial period of the intensity distribution of the diffraction interference pattern 13 formed on the light receiving surface.
According to the diffraction theory of light, an intensity pattern similar to the diffraction grating pattern of the scale 2 is formed on the light receiving surface of the photosensor 3, if z1 and z2 defined as above meet the relationship given below:(1/z1)+(1/z2)=λ/kp12  (1)                 where λ is the wavelength of the light beam emitted from the light source, and k is an integer.        
In this case, the pitch p2 of the diffraction interference pattern 13 on the light receiving surface can be expressed by an equation (2) given below by using the other structural parameters:p2=p1(z1+z2)/z1  (2) 
If the scale 2 is displaced with respect to the light source 1 in the pitch direction of the diffraction grating, the intensity distribution of the diffraction interference pattern 13 is moved in the direction of displacement of the scale while maintaining the same spatial period.
Therefore, if the spatial period p20 of a light receiving area 4 of the photosensor 3 is set at a value equal to that of p2, a periodic intensity signal is obtained every time the scale 2 is moved by p1 in the pitch direction, making it possible to detect the amount of displacement of the scale 2 in the pitch direction.
A prior art relating to a small displacement sensor using a surface emitting laser light source will now be described as a second prior art.
The small displacement sensor is a complex resonator type interference sensor using a surface emitting laser light source, which is disclosed in a paper by the present inventors, entitled “Ultra small sensor using a surface emitting laser” by Eiji Yamamoto, Lecture Materials (VI) for Machinery Institute 75th Periodic General Meeting, 1998, pp. 682-689.
As shown in FIGS. 30A and 30B, this small displacement sensor has a surface emitting laser light source 10 and an outer mirror 61 positioned to face each other so as to constitute a complex resonator. The light emitted from the surface emitting laser light source 10 is detected by the light receiving area 4 formed in the photosensor 3 so as to detect a change in the distance L between the surface emitting laser light source 10 and the outer mirror 61.
According to this paper, the output characteristics of the sensor when the distance L is changed depend on many structural parameters. However, typical examples of calculation are shown in the case of using an edge emitting type semiconductor laser which is an ordinary conventional semiconductor laser widely used (FIG. 31A), in comparison with the use of a surface emitting laser (FIG. 31B).
According to the paper, when an edge emitting type semiconductor laser is used as the light source, the laser output is scarcely changed regardless of a change in the distance L between the light source and the outer mirror, if the distance L is at least several scores of μm. When a surface emitting laser is used as the light source, by way of contrast, a slight change in the distance L greatly changes the laser output even if the light source and the outer mirror are positioned apart from each other by over several millimeters.
The paper also teaches that, even in the case of using a surface emitting laser as the light source, the laser output is scarcely changed regardless of a change in the distance L, if the distance L is set relatively large by inclining the outer mirror facing the light source.
FIG. 31C shows the characteristics in the case where the outer mirror is inclined by 0.5° with the structure similar to that shown in FIG. 31B. As also apparent from FIG. 31C, it is known that a change in the laser output with respect to a change in the distance L can be suppressed by further tilting the outer mirror even if the distance L is small.
Let us consider the case where z1 and z2 deviate from the relationship given by the equation (1) because of an initial misalignment in assembling the sensor and the mechanical swinging caused by the displacement of the scale in the prior arts shown in FIGS. 28 and 29A to 29E.
For example, where the scale position is deviated by Δz from the position of the scale 2 to the position of a scale 22 as shown in FIG. 29A, though the light source and the light receiving surface are fixed, the diffraction interference pattern on the light receiving surface is disturbed as shown in FIGS. 29B and 29C as well as the pitch p2 of the diffraction interference pattern 13 on the light receiving surface is changed according to the equation (2).
It is to be noted that the expression “the diffraction interference pattern 13 on the light receiving surface is disturbed” implies that the similarity of the diffraction interference pattern 13 on the light receiving surface to the diffraction grating pattern of the scale 2 is disturbed.
In the conventional structures shown in FIGS. 28 and 29A to 29E, z1 is z1+Δz and z2 is z2−Δz in the ordinary case where the arrangement of the light source 1 and the photosensor 3 is fixed.
Let us now consider the case where the scale surface and the light receiving surface are arranged in parallel.
If the pitch of the interference pattern formed on the light receiving surface is changed from p2 to p2′ when a positional deviation Δz has taken place, the following equation (3) is satisfied:                                                                                                          p2                  ′                                =                                ⁢                                                      p1                    ⁡                                          (                                              z1                        +                                                  Δ                          ⁢                                                                                                           ⁢                          z                                                +                        z2                        -                                                  Δ                          ⁢                                                                                                           ⁢                          z                                                                    )                                                        /                                      (                                          z1                      +                                              Δ                        ⁢                                                                                                   ⁢                        z                                                              )                                                                                                                          =                            ⁢                                                p1                  ⁡                                      (                                          z1                      +                      z2                                        )                                                  /                                  (                                      z1                    +                                          Δ                      ⁢                                                                                           ⁢                      z                                                        )                                                                                        (        3        )            
Therefore, if a plurality of light receiving regions 4 of the photosensor 3 are formed in accordance with the period of the pitch p2, a deviation between the period of the light receiving region and the period of the diffraction interference pattern 13 is increased at the position away from the principal axis of the beam from the light source. As a result, as shown in FIGS. 29D and 29E, it is inevitable that the amplitude of the output signal Ipd from the photosensor is lowered and the diffraction interference pattern 13 is disturbed.
Suppose that z1=0.5 mm, z2=0.5 mm and                Δz=−z1/10=0.05 mm. In this case, p2=20 μm and p2′=22.2 μm are obtained from an equation (3).        
Therefore, even if the pitch of the light receiving area 4 is set at p20=p2=20 μm as designed, 4.5p2=4.0p2′ at the position away from the principal axis of the light beam on the light receiving surface by Sx/2=4.5p2=90 μm, with the result that the diffraction interference pattern 13 is deviated by ½ pitch. It follows that a signal 14 output from the light receiving area 4 at this position bears a reversed phase, leading to reduction in the amplitude of the output from the sensor.
In this case, the phase of the diffraction interference pattern 13 is reversed on the light receiving surface at the position where the apparent angle θ when viewed from the light source becomes 2Arc Tan (4.5p2/(z1+z2))=10.3°. With this angle taken as the maximum apparent angle θmax, it is desirable to set the distribution width Sx of the light receiving area 4 at a value corresponding to about half of θmax representing the maximum apparent angle and also to set the beam spread angle of the coherent light at about θmax.
The distribution width Sx of the light receiving area noted above implies the entire expansion where the aforementioned plural light receiving regions are distributed.
To be more specific, in order to suppress the reduction in the amplitude of the output signal Ipd caused by the positional deviation of the scale and a variation in the assembling step of the sensor and to obtain the appropriate light receiving level, it is effective to limit the distribution width Sx of the light receiving area to an area in the vicinity of the principal axis of the light beam and to use a coherent light source having a spread angle of the light beam corresponding to the distribution width.
Under the circumstances, in the case of the conventional structure which uses an edge emitting type semiconductor laser as the light source, the spread angle of the light beam is very large, i.e., about 40° along the longer axis and about 20° along the shorter axis, making it difficult to emit a laser beam having a beam spread angle θmax of about 10°. What should be noted is that most of the laser beam spread by an angle larger than the θmax of about 10° causes a significant reduction in the amplitude of the sensor output or leads to a lower light receiving level.
Such being the situation, what is required is a sensor using a coherent light as the light source that permits the spread angle of the laser beam emitted from the light source to be set appropriately.
Further, even if the light receiving area is limited to a region in the vicinity of the principal axis of the light beam, it is unavoidable for the period of the output signal Ipd to change, leading to an error in the measurement of the absolute value in the amount of displacement of the scale.
The requirement to suppress the change in the period of the output signal Ipd when a positional deviation Δz (Δz means a gap variation between the scale and the head) has taken place in the scale is the structure that does not bring about a change in the period of the diffraction interference pattern on the light receiving surface.
Another problem inherent to the prior art is that the light emitted from the laser light source is reflected on the surfaces of the scale and the photosensor so as to return to the laser light source, which results in a change in the light intensity occurs, leading to noise generation in the output signal.
A measure against the difficulty is essential particularly in a case where a laser having a small beam spread angle such as a surface emitting laser is used as the light source, as already described in conjunction with the second prior art. For suppressing the difficulty, it is necessary to provide the structure that can lower the noise caused by the returning laser light beam.
The following is the summary of the shortcomings of the above-described prior arts and the subject matters of this invention to cope with the shortcomings.
As the prior arts use a conventional semiconductor laser as the light source, the spread angle of the light beam is very large, i.e., about 40° in the direction of the longer axis and about 200 in the direction of the shorter axis. In addition, it is impossible to set the spread angle of the light beam as desired. Therefore, if the expansion of the light receiving area is limited to a region in the vicinity of the principal axis of the light beam, the power of the light incident to the light receiving area is remarkably lowered, resulting in failure to solve the problem that the S/N ratio of the signal is lowered.
A first subject matter of this invention, which has been achieved in view of the situation described above, is to provide the structure that permits setting the spread angle of the light beam to be set to a predetermined small angle that cannot be achieved by the conventional semiconductor laser light source and to realize an optical displacement sensor that provides an output signal of a good S/N ratio even if the arrangement of the light source, the scale and the light receiving element deviates from the optimum arrangement.
In the prior arts, the surface of the scale and the light receiving surface of the photosensor are arranged perpendicular to the principal axis of the light beam emitted from the laser light source. As a result, the light emitted from the laser light source is reflected on the surfaces of the scale and the photosensor so as to return to the laser light source, thus generating noise.
A second subject matter of this invention, which has been achieved in view of this situation, is to provide an optical displacement sensor that prevents the laser light from returning to the light source to be thereby able to suppress the superimposition of the noise caused by the returning laser light on the output signal of the sensor.
Further, in the case where the arrangement of the light source, the scale and the light receiving element deviates from the designed layout in the prior art, the period and the position of the diffraction interference pattern on the light receiving surface are greatly changed, resulting in a failure to suppress the reduction in the amplitude of the signal and the change in the period with respect to the scale displacement.
A third subject matter of this invention, which has been achieved in view of this situation, is to provide an optical displacement sensor capable of reducing changes in the period and position of the diffraction interference pattern on the light receiving surface and also capable of suppressing the reduction of the signal amplitude and change in the period with respect to the scale displacement.
Furthermore, this invention is directed to a fourth subject matter in addition to the first to third subject matters described previously.
Specifically, the fourth subject matter of this invention is to provide an encoder that has the reference point detecting function and the absolute point detecting function and also can accurately detect the displacement of the scale in the x direction while being scarcely affected by the change in the gap between the scale and the head.
A fifth subject matter of this invention is to reduce the assembling cost by employing a mounting mode that does not use an inclined substrate.
Further, a sixth subject matter of this invention is to provide a structure and means that stabilize the encoder output signal in spite of a change in the environment.